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Thursday, 6 September 2012

nature

In 2008, a study proposed that the maximum height for a Doug­las fir
-- one of the world's tallest trees -- is about 453 feet (138 meters)
[source: Kinver]. But why is there a limit? Trees are sup­posed to be nature's skyscrapers, impossible to hem in. This cap exists because trees can only pull water so far up their trunks.The transporters in question are pitted dead cells, called tracheids,
that move water from one cell to the next. The diameter of these
all-important pits shrinks as you ascend into a tree's upper reaches,
complicating water transport. Eventually the flow of water reaching the
leaves and branches near the top dwindles off or stops, and the
struggling sections experience "drought stress." They become dehydrated
and die, establishing a de facto maximum height for that tree